The present invention relates to electric cables, intended in particular for medium voltage and high voltage networks, and required to remain leakproof over long lifetimes.
Single-conductor cables of this type are generally of the structure shown diagrammatically in the accompanying figure. The central conductor 10 is made up of a bundle of wire strands that are often twisted helically and that are generally made of copper or aluminum, and it is coated in a coating 12 of so-called xe2x80x9csemiconductivexe2x80x9d material intended in particular for smoothing the outside surface of the bundle and consequently for reducing peaks in electric field. A layer 14 of insulating material, generally made of polyethylene or peroxide-cured ethylene-propylene rubber (EPR) covers the assembly constituted by the conductor and the coating. It is in turn generally covered by a second coating of semiconductive material 16 referred to as the xe2x80x9couterxe2x80x9d semiconductor. The three layers built up in this way are generally cured (i.e. cross-linked) by peroxide decomposition at high temperature; they can also be cured by the silane method.
The resulting assembly is often referred to as an electrical core and it is protected against moisture which would run the risk of giving rise to electrical treeing or arcing. For this purpose, a conductive tube 18 generally made of aluminum or copper is used that generally has a thickness of one to a few hundred microns (xcexcm), which tube also constitutes an electric shield. The tube is often constituted by foil or tape that is folded or wound around the insulation with overlapping margins, and by applying adhesive, e.g. using a hot-melt adhesive. The metal tube can also be constituted by an extruded metal, e.g. lead or aluminum. Finally, a mechanical protective sheath 20 of polyethylene or polyvinyl chloride type material that combines good mechanical characteristics with good resistance to cracking and good resistance to thermal aging, is then stuck on the screen. Often a sealing powder is placed on the coating 16 (which then has external fluting) prior to the tube being put into place. Another solution consists in using a smooth coating 16 on which a swelling tape is placed, which tape may be conductive, followed by the metal tube. Wires can also be added if the section of the metal screen is not sufficient to carry short-circuit currents. Combinations of these various components can also be used.
Such cables are used in particular for distribution at medium voltage, i.e. in the range 12 kilovolts (kV) to 20 kV.
In numerous applications, it is desirable to make the semiconductive coating peelable, i.e. separable from the insulation on which it is to be found by applying a small amount of force, generally less than 25 Newtons per centimeter (N/cm) at a temperature of 20xc2x0 C. The use of such an outer semiconductor makes it easier to prepare the ends of a cable for splicing or termination purposes.
Various compositions are already in use for making such outer semiconductors. In particular, the following composition is used:
ethylene vinyl acetate copolymer (EVA) having a vinyl acetate content in the range 40% to 45%, such a polymer serving to absorb carbon black by means of its acetate groups and limiting adhesion on the layer 14 of insulating material;
synthetic acrylonitrile butadiene rubber, known as NBR, whose function is to reduce the force of adhesion on the layer of insulating material to the desired value which generally lies in the range 5 N/cm to 25 N/cm at 20xc2x0 C.; and
conductive carbon black, whose content generally lies in the range 40% to 60% of the elastomer content.
The composition often also includes lubricants, antioxidants, and a curing agent for cross-linking the copolymers, which agent can be constituted by peroxides.
The above composition gives good technical results. However its cost is high because it uses EVA having 40% to 45% vinyl acetate, which is not widely available and which is expensive. This type of EVA copolymer can be obtained only by a manufacturing method that is complex, making use of polymerization in solution.
Consequently, it is an object of the present invention to provide a vulcanizable semiconductive composition presenting electrical, mechanical, aging, and adhesion characteristics that are comparable to those of the best compositions known in the past, but using a copolymer whose monomer content is less than 40%, thus enabling it to be fabricated by a high pressure method, similar to that used for polyethylene, and thus more widely available and of much lower cost.
Conductive compositions, e.g. using ethylene and vinyl acetate copolymers with EVA contents of less than 40% have already been described, e.g. in document EP 0 420 271 B1, but those formulations are rather constricting, in particular having an EVA copolymer content greater than 40%, with limits on the specific surface area of the carbon black, which must lie in the range 30 square meters per gram (m2/g) to 60 m2/g, and an additive content (i.e. of plasticizers, lubricant, inert fillers, antioxidant, anti-UV) which must be less than 3%.
The proposed formulas are not suitable for achieving peeling forces of less than 40 N/cm, whereas it is desirable to achieve forces of less than 40 N/cm and preferably lying in the range 5 N/cm to 25 N/cm in order to make it easy to prepare cable splices and terminations.
For this purpose, there is provided in particular a cross-linkable composition comprising:
a mass of copolymer constituted by:
a copolymer of ethylene and an organic acid salt of an alkyl, alkenyl, or alkynyl radical comprising 26% to 42% of weight of monomer, representing 50% to 90% by weight of the polymer phase; and
an acrylonitrile and butadiene copolymer known as NBR having 25% to 50% by weight acrylonitrile, representing 10% to 50% by weight of the polymer phase;
carbon black having a specific surface area of less than 170 m2/g, typically representing 40% to 75% by weight of the polymer phase.
Typically there will be additives such as antioxidants, anti-coppers, peroxides, silanes, in quantities that generally remain below 5% of the polymer phase.
Optionally, the composition has one or more of:
at least one aliphatic, aromatic, or naphthenic plasticizer whose content by weight is advantageously less than 20% of the polymer phase;
lubricants capable of belonging to the stearate, oleate, amide, polyethylene wax, or silicone families, at a content generally less than 10% of the polymer phase; and
inert mineral fillers (chalk, kaolin, alumina, talc, silica) at a content that is generally less than 50% of the polymer phase.
The copolymer can, in particular, be one of the following which have been found to be particularly advantageous:
EVA: ethylene vinyl acetate which has been found to be particularly advantageous;
EBA: ethylene butyl acrylate;
EMA: ethylene methyl acrylate; and
EEA: ethylene ethyl acrylate.
In practice, the alkyls used are generally lower alkyls (having up to five carbon atoms).
In terms directly of weight, formulations of the invention will generally have the following composition:
25% to 40% EVA (ethylene vinyl acetate) copolymers, as described above;
7% to 25% NBR (acrylonitrile butadiene rubber) copolymers, as described above;
19% to 40% carbon black presenting a specific surface area of less than 170 m2/g, as measured using the standard ASTM D 4820; and
0.4% to 3% additives such as peroxides and antioxidants.
The following can be added thereto:
0 to 25% inert filler (chalk, kaolin, talc, silica, alumina, etc.);
0 to 20% aliphatic, aromatic, or naphthenic plasticizer; and
0 to 10% lubricants.
A marked advantage of formulations of the invention is that they use copolymers that are widely available and less expensive.
The particular semiconductive formulation that is selected will, technically speaking, typically be a compromise between characteristics enabling it to be used on an industrial scale. Frequently, the term xe2x80x9cconductivexe2x80x9d will be use rather than xe2x80x9csemi-conductivexe2x80x9d for short and also to avoid confusion with materials designated as semi-conductors in the elctronic industry.
This compromise will take account of the following in particular:
ease with which the material can be worked, expressed both in terms of viscosity which must be low enough to prevent the material self-heating during extrusion, and in terms of roasting time which must be long enough to avoid any chance of the cross-linking reaction starting within the extrusion tooling;
reactivity, which expresses the reaction rate of the material in the cross-linking process;
resistivity which must be low enough to ensure that the extruded layer performs its function as a conductive screen effectively;
mechanical characteristics which must be sufficient to enable the material to be peelable without tearing or breaking;
peelability, which must lie between upper and lower limits so that the conductive layer can be separated easily from the insulation but without any risk of unwanted separation occurring between the insulation and the conductive layer, which would harm the electrical function of the cable. Peelability lying in the range 5 N/cm to 25 N/cm at 20xc2x0 C., as measured by peeling through 180xc2x0 between the insulation and the conductive layer in application of standard NFC 33223 enables cables to be prepared easily without harming their electrical properties; and
the surface appearance of the insulation after the conductive layer has been peeled off, which appearance must be sufficiently smooth and should not present any traces of the conductive composition since that would harm the electrical quality of a connection or termination.
Often a carbon black content lying in the range 27% to 35% and/or having a specific surface area lying in the range 30 m2/g to 80 m2/g will give the best results. The ethylene copolymer content will frequently lie in the range 30% to 35% of the total composition.
The invention also provides a method of manufacturing an insulated cable in which a triple head is used to coat a solid or cabled electrical conductor simultaneously with an inner semiconductor, insulation, and an outer semiconductive layer having a composition of the type defined above, and all three layers are cross-linked immediately after extrusion by passing the coated conductor through a heated tube under gas pressure.
The peroxide and the antioxidant required for cross-linking the insulation, e.g. based on polyethylene, can be injected in liquid form at the time of extrusion. In particular, the peroxide can be dicumyl peroxide, butylcumyl peroxide, or ditertiobutyl peroxide. The peroxide for cross-linking the semi-conductive layer can be injected into the extruder at the time the cable is made. This provides an electric cable having a central conductor coated in a coating of semiconductive material, a layer of insulating material, an outer layer of semiconductive material in accordance with the invention, a metal screen made of wires or tapes, and an outer protective sheath. The cable is suitable for being assembled in the form of a bundle of three conductors, possibly together with a carrier.
The above characteristics and others will also appear on reading the following description of comparative examples making it easier to understand the advantages of composition of the invention.